Descaling of metal strip, especially stainless steel strip, has taken many forms in the past. The simplest technique involves only the pickling of the strip in mineral acid such as sulfuric acid, hydrochloric acid, hydrofluoric acid, nitric acid, or mixtures thereof. This may work with some grades of stainless steel with very light scale, however, in most cases more is needed than just an acid pickle. In those cases, various compositions and techniques have been developed to condition the scale before acid pickling. Typical compositions for scale conditioning include mixtures of alkali metal hydroxides and alkali metal nitrates with various other additives, such as alkali halides, carbonates, and/or other oxidizing agents. These are often referred to as descaling or scale conditioning salts. A conventional technique for using such compositions is in the fused anhydrous state in a pot at elevated temperatures, e.g. 800° F. to 1000° F., through which the strip is passed, followed by an acid pickle. While this works well in many cases, nevertheless there are certain drawbacks to this technique in some instances. For example, the bath has to be maintained at elevated temperatures, which may be energy intensive. Also, the fused caustic baths require submerged rolls which may be difficult to maintain, and can cause marring of the surface of the strip being descaled. Additionally, there is the issue of drag-out of the fused composition, i.e. as the strip exits from the pot of fused composition, it carries a certain amount of the fused composition with it, especially at high strip speeds. Furthermore, fused bath compositions are limited to compounds that have long term stability at elevated temperatures.
Other techniques for descaling are disclosed in commonly assigned U.S. Pat. No. 3,126,301, issued Mar. 24, 1964, entitled “Molten Salt Spray Process for Descaling Stainless Steel” and U.S. Pat. No. 5,272,798, issued Dec. 28, 1993, entitled “Method and Apparatus for Descaling Metal Strip”. These patents disclose methods and structures for spraying fused caustic-containing compositions onto a moving strip of steel to condition the scale, after which the scale is pickled off. These offer several advantages, in some instances, over the technique of using a pot of fused material. However, they too have some drawbacks in some instances. They require high temperature nozzles, and the composition must be maintained at elevated temperatures, e.g. 800° F. to 1000° F.
Thus, there is a need for a relatively inexpensive, low temperature, and efficient technique for conditioning scale on metal surfaces, especially on stainless steel strip, or the like.